1. Field of the Invention
The present invention relates to a correction apparatus and a correction method for correcting an error of an angle sensor that generates a detected angle value having a correspondence with an angle to be detected, and to an angle sensor including the correction apparatus.
2. Description of the Related Art
In recent years, angle sensors have been widely used to generate a detected angle value having a correspondence with an angle to be detected in various applications, such as detection of the rotational position of a steering wheel or a power steering motor in an automobile. Examples of the angle sensors include a magnetic angle sensor. A system using the magnetic angle sensor is typically provided with a magnetic field generator for generating a rotating magnetic field whose direction rotates in response to the rotation or linear movement of an object. The magnetic field generator is a magnet, for example. The angle to be detected by the magnetic angle sensor is, for example, the angle that the direction of the rotating magnetic field in a reference position forms with respect to a reference direction.
U.S. Patent Application Publication No. 2012/0095712 A1 describes a rotating field sensor, which is a magnetic angle sensor. The rotating field sensor includes a signal generator for generating first and second signals associated with the intensities of components of a rotating magnetic field in mutually different directions, and an angle detector for calculating a detected angle value based on the first and second signals. The signal generator includes a first detection circuit for outputting the first signal, and a second detection circuit for outputting the second signal. Each of the first and second detection circuits includes at least one magnetic detection element. The magnetic detection element is, for example, a spin-valve magnetoresistance (MR) element including a magnetization pinned layer whose magnetization direction is pinned, a free layer whose magnetization direction varies depending on the direction of the rotating magnetic field, and a nonmagnetic layer located between the magnetization pinned layer and the free layer.
Ideally, when the direction of the rotating magnetic field changes with a constant angular velocity, the first signal and the second signal in the rotating field sensor should have sinusoidal waveforms (including sine waveforms and cosine waveforms) that are different in phase from each other by 90°. However, as described in U.S. Patent Application Publication No. 2012/0095712 A1, the first and second signals may have waveforms distorted from a sinusoidal curve. When the first and second signals have distorted waveforms, the first signal contains a first ideal component which varies in such a manner as to trace an ideal sinusoidal curve, and a first error component other than the first ideal component, and the second signal contains a second ideal component which varies in such a manner as to trace an ideal sinusoidal curve, and a second error component other than the second ideal component.
One of the causes of the distortion of the waveforms of the first and second signals is a variation of the magnetization direction of the magnetization pinned layer of the MR element due to the influence of the rotating magnetic field or the like. The distortion of the waveforms of the first and second signals may result in some error in the detected angle value.
U.S. Patent Application Publication No. 2012/0095712 A1 discloses a technique for reducing an error that occurs in the detected angle value. In the technique, a square-sum signal made up of the sum of the square of the first signal and the square of the second signal is generated, and the first and second signals are corrected on the basis of the generated square-sum signal.
U.S. Patent Application Publication No. 2006/0076480 A1 discloses a technique for correcting two-phase sinusoidal signals. When two-phase sinusoidal signals out of phase with each other are output from an encoder, they may form a Lissajous waveform containing an error deviating from an ideal Lissajous waveform. In this technique, such an error is detected and the two-phase sinusoidal signals are corrected on the basis of the detected error. The two-phase sinusoidal signals in U.S. Patent Application Publication No. 2006/0076480 A1 are equivalent to the first and second signals in U.S. Patent Application Publication No, 2012/0095712 A1. A radius of the Lissajous waveform in U.S. Patent Application Publication No. 2006/0076480 A1 is equivalent to a square root of the square-sum signal in U.S. Patent Application Publication No. 2012/0095712 A1. In the following description, the two-phase sinusoidal signals in U.S. Patent Application Publication No. 2006/0076480 A1 will also be referred to as the first and second signals.
Both of the techniques disclosed in U.S. Patent Application Publication Nos. 2012/0095712 A1 and 2006/0076480 A1 relate to correction performed to reduce fluctuation in the magnitude of the square-sum signal. These techniques can thus reduce an error that causes fluctuation in the square-sum signal.
While errors occurring in a detected angle value from an angle sensor vary depending on the angle to be detected, some of the errors do not cause fluctuation in the magnitude of the square-sum signal. Such an error will hereinafter be referred to as angle-dependent error. The angle-dependent error results from errors occurring in the same phase in the first signal and the second signal. More specifically, the angle-dependent error occurs, depending on the angle to be detected, when the first signal and the second signal deviate from the first ideal component and the second ideal component, respectively, by the magnitude corresponding to the angle-dependent error. For example, the angle-dependent error occurs when the free layer of the MR element in the first detection circuit and the free layer of the MR element in the second detection circuit have magnetic anisotropies in the same direction, or when there is a misalignment of relative positions of the magnetic field generator and the signal generator with respect to each other. Neither of the techniques disclosed in U.S. Patent Application Publication Nos. 2012/0095712 A1 and 2006/0076480 A1 can reduce the angle-dependent error.